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The Photoelectric Effect and the Quantization of Light

Introduction

When a light with a sufficiently high frequency shines on a metal plate, electrons are ejected from the plate. This effect is known as the photoelectric effect. The electrons ejected in this process are called photoelectrons.
Figure 1

Figure 1

There are several important features of the photoelectric effect that cannot be explained by the classical theory of electromagnetic waves. These are the following. In order to explain the photoelectric effect, Einstein proposed in 1905 that light of frequency f carries energy in discrete packets, each packet containing an amount of energy, E, given by:
( 1 )
E = hf 
where h is a constant now called Planck's constant. The value of Planck's constant was determined experimentally to be the following.
( 2 )
h = 6.6260755 × 10–34 J · s
 
Today the energy packets are called photons. When light is incident on a metal surface, an electron in the metal can absorb the photon, thus the photon transfers its energy and momentum to the electron. If the electron acquires enough energy from the photon, it can escape from the metal. Since the electron is bound to the metal, an amount of energy is required to escape the metal. The amount of energy needed for the least tightly bound electron to escape is known as the work function, ϕ. The value of ϕ depends on the specific metal surface being illuminated and is typically on the order of a few electron volts (eV). Therefore, according to Einstein's photoelectric theory (which earned him a Nobel prize), an electron can only be ejected if the photon's frequency is larger than a threshold value, fo, given by the equation below.
( 3 )
hfo = ϕ
 
If the incident photon has energy greater than ϕ, then the excess energy becomes the kinetic energy of the electron. The electrons with the largest kinetic energy are those that are the least tightly bound. Since ϕ is the energy required for the least tightly bound electron to escape, the maximum kinetic energy is given below.
( 4 )
KEMAX = hf – ϕ
 
The maximum KE of the ejected electrons, therefore, increases with increasing light frequency and is independent of the light intensity, as observed experimentally. However, the number of photons striking the metal surface increases linearly with the light intensity. Thus, the number of electrons ejected is proportional to the intensity of the incident light.

Apparatus

Figure 2

Figure 2

The apparatus for this experiment has three essential parts: a high intensity mercury light source that provides photons of different frequencies, a diffraction grating/lens system to spatially separate and focus the light or photons with different frequencies, and the target, which is the anode of a vacuum phototube that is housed in the h/e apparatus together with the associated electronics. The arrangement of the three parts is shown in Fig. 2. When the light from the mercury lamp passes through the lens/grating assembly, it produces a spectrum as shown in Fig. 3. Note that the spectrum is not symmetric on the left and right sides because the diffraction grating is blazed to enhance the first-order pattern on one side. The frequencies and the wavelengths of the spectral lines are shown in the table below.
Figure 3

Figure 3

In this experiment, light from each spectral line is focused on a phototube as shown in Fig. 4. The phototube is a vacuum tube with a semi-cylindrical metal electrode (anode) and a thick metal wire at the center (cathode). When a photon with sufficient energy strikes the anode of the phototube, an electron is released and collected at the cathode (see Fig. 4). The process leaves the anode with a positive charge and a positive potential with respect to the cathode (which is grounded). As more electrons are ejected from the anode it becomes more positively charged. The positive charges attract the photoelectrons as they try to leave the anode, thus causing them to lose kinetic energy as they move to the cathode. The amount of kinetic energy lost per electron is equal to eV, where e is the charge of the electron and V is the potential between the anode and the cathode. When V becomes large enough such that
eV = KEMAX,
even the most energetic electrons (with maximum KE) will be attracted back to the anode and hence, V will have reached a maximum. This maximum value of V is called the stopping potential. Thus, if we can measure this potential and substitute eV for
KEMAX
in Eq. (3)hfo = ϕ
 
, we get the following.
Figure 4

Figure 4

( 5 )
eV = hf - ϕ
 
Dividing Eq. (5)eV = hf - ϕ
 
by e we get the following.
( 6 )
V =
h
e
f
ϕ
e
 
A plot of the measured stopping potential as a function of the frequency of the photon should be a straight line (see Fig. 5) whose slope gives Planck's constant h divided by e. The intercept gives the work function ϕ divided by e, which is 1.602 × 10–19 C.
Figure 5

Figure 5

To facilitate measurement of the stopping potential, the anode is connected to a built-in amplifier with an ultra-high input impedance (> 1012 Ω). The output from this amplifier is connected to the output jacks on the front panel of the apparatus. This high impedance, unity gain (
VOUT/VIN = 1
) amplifier lets you measure the stopping potential with a digital voltmeter. While the impedance (i.e. resistance) of the amplifier is very high, it is not infinite and some charge leaks off. Thus charging the apparatus is analogous to filling a bathtub with different water flow rates while the drain is partly open.

Procedure

Part 1: Apparatus Set Up

The apparatus should be mostly assembled when you arrive. You may, however, need to check focus and alignment and adjust these as needed. See Fig. 2 for overall assembly view.
Do not turn off the lamp unless told to do so by your instructor.
Figure 4

Figure 4

It is important that light of only one color enters the slot. There must be no overlap from adjacent spectral maxima.
Figure 7

Figure 7

Part 2: Intensity Dependence

According to the quantum model of light, the maximum kinetic energy,
KEMAX
, of photoelectrons depends only on the frequency of the incident light, and is independent of the intensity. Thus, the higher the frequency of the light, the greater its energy. In contrast, the classical wave model of light predicted that
KEMAX
would depend on light intensity. In other words, the brighter the light, the greater its energy. In this part of the experiment, you will investigate the intensity dependence of the photoelectric effect. You will select two spectral lines from the mercury light source and investigate the maximum energy of the photoelectrons as a function of the intensity.
The electrodes of the phototube should be discharged before each measurement by using the discharge button, otherwise the potential difference reading may be inaccurate.
Note: a small drop (a few percent or less) in potential difference with decreasing light intensity is most likely due to charge leaking off the high impedance amplifier in the h/e Apparatus. The charging time should increase with decreasing intensity, but do not be concerned if this is not the case since the charge leaking from the amplifier may alter the results.

Part 3: Stopping Potential Dependence on Frequency

In this part of the experiment you will measure the stopping potential for different frequencies of light incident on the phototube.

Analysis

Discussion

Part 2: Intensity Dependence

Summarize your experimental results for the intensity dependence of the stopping potential. Describe the effect that passing different amounts of the same colored light through the Variable Transmission Filter has on the stopping potential, and thus, the maximum energy of the photoelectrons. Defend whether this experiment supports a wave or a quantum model of light based on your lab results.

Part 3: Stopping Potential Dependence on Frequency

Summarize your experimental results for the Planck's constant, the work function and the threshold frequency. Compare your value of h to the accepted value of 6.626 × 10–34 J · s. Discuss possible systematic errors and how they might affect your results.
Be sure that both you and your TA each initial your data, and that you hand in a copy of your data before leaving the lab. Remember to pledge your work.